Due to popular demand the TubeSync Bias Engine is now in volume production!
For order enquiries please email sales@tubesync.co.uk
Posts Tagged ‘biasing’
TubeSync Bias Engines Roll Off The Production Line
Friday, August 20th, 2010
Never Hang Your Guitar Tubes / Valves Upside Down
Thursday, March 18th, 2010
Never hang your guitar tubes upside down as many horrible things may happen!!!
The heat from the electrodes will go directly to the base pins and cause problems with oxidization. The heat will also cause micro-cracks on the pins of the tube / valve, which could result in failures.
So don’t do it !!…….
Hiwatt Underline Reliability Repuation With TubeSync
Thursday, February 18th, 2010
Famed for their legendary reliability and tone, Hiwatt have gone one step further in the reliability stakes by employing TubeSync® technology in their already bulletproof amplifiers.
Over the last year, Hiwatt and UK based KBO Dynamics have developed their own version of a pioneering technology called TubeSync®, which Hiwatt will launch at Frankfurt Musikmesse 2010 in March. The technology has been developed in order to overcome problems associated with conventional tube amplification such as biasing, tube longevity and overall reliability. What the collaboration has achieved is to improve amplifier design without changing the fundamental amplification process yet retaining the classic Hiwatt tone and sound.
Although not an integral part of the amplification process, Tubesync® is connected to the heart of the amp’s circuitry, constantly testing and monitoring; it’s like having an amp technician working full time inside the box every time the amp is powered up. This technology eliminates the need for tube bias current matching, increases tube life by micro-adjusting the bias on each tube, predicts tube failure and can, in the event of a catastrophic tube failure, even run the amp at half power. The result is the eradication of many of the problems hampering conventional tube amplifiers and peace of mind for the musician wise enough to have taken this route.
KBO Dynamics Chief Exec Andy Fallon: “It has been a pleasure working with Hiwatt who have been extremely receptive to change and have embraced the new technology. They have seen the benefits it can bring to them and of course, their customers, and we are looking forward to further exciting collaborations. This technology genuinely has raised the bar concerning reliability and classic tube amps, and what’s more. It’s been developed exclusively in the UK .”
For more information on TubeSync® technology, please visit www.kbodynamics.com and www.hiwatt.co.uk.
Hiwatt produce classic British custom amps both guitar and bass.
Tube Failure Modes, Within Amplifiers
Tuesday, August 11th, 2009
Catastrophic Failures. These are failures that occur suddenly, without warning, making the equipment unusable.
Examples of catastrophic failures are: -
- Glass failure, loss of vacuum due to mechanical damage or thermal stress.
- Heater failures open circuit or partial short circuit due to excessive heater voltage or high initial surge current and normal on off cycling over many thousands of hours.
- Arcing, due to low cathode temperature, causing damage to the cathode and grids.
- Bias failure due to component leakage or valve characteristic spreads using fixed or auto bias.
Degenerative Failures. The slow but eventual, deterioration of all tubes, which can contribute to the end of life of the tube.
Examples of degenerative failures: -
- Gas is present in all tubes and if the tube is used within its characteristics should not be a problem, however excessive dissipation can liberate gas from the tube structure and lead to eventual premature failure.
- Getters are patches of evaporated metal, which are deposited on the inside of the glass. The purpose of the getter is to absorb any gas that may be evolved during the life of the tube and work best at normal glass bulb temperatures.
- Spurious emissions are uncontrolled unwanted emissions usually caused by gas released due to excessive dissipation and elevated temperatures.
- Inter electrode leakage. This is current, which flows between the electrodes of the tube, which are not connected in any way. The cause is metallic vapours released by the hot metallic structure of the valve being deposited on the insulating micas and eventually leading to a conductive path.
- Cathode depletion can occur due to arcing, which can strip the cathode coating reducing the active area of the cathode. Arcing will vaporise the cathode material and generate gas, which can poison the cathode material. This can also be caused, by passing excessive cathode current before the heater has reached its normal operating temperature.
Subjective Failures. These are tubes, which will normally pass tests but do not perform satisfactorily due to for example: – hum level, microphonics and noise.
Examples of subjective failures: -
- Hum is an unwanted mains frequency signal, which is superimposed on the wanted signal. It can be caused by heater to cathode leakage or due to electrostatic or electromagnetic fields within the equipment.
- Microphonics is defined as a signal originating inside the valve caused by mechanical vibrations being amplified by the tube.
- Noise is a signal originating from inside the tube, but not due to hum or microphonics. It can be due to intermittent short circuits, open circuits or arcing due to leakage paths between the electrodes.
Characteristic Variables. These are variations or spreads in the tube characteristics, due to manufacturing tolerances and follow a normal distribution curve.
Examples of characteristic variables: -
- The Standard is usually a tube that complies with the manufactures published data. The Upper and Lower limits are values that any tube within these limits can be considered acceptable for normal use.
- Characteristic Spreads is the degree of deviation from the standard tube.
- Design Tolerances is the normal variations of standard parts, which a well-designed piece of equipment will operate correctly.
The Solution Is ……….TubeSync!
TubeSync Amp Biasing
www.kbodynamics.com
Conventional Tube Biasing
Wednesday, July 8th, 2009
A very important feature of any thermionic tube circuit is known as ‘biasing’. It has already been mentioned that a negative voltage applied to the control grid of the valve will restrict or prevent the current flowing between anode and cathode. This is also known as a ‘bias’ signal, as it is used to control the amount of current which flows under ‘no-audio-signal’ conditions (quiescent conditions).It is common practice to operate tube amplifiers in a state known as ‘Class A-B’. This means that when no signal is present, a relatively small amount of D.C. current is allowed to flow in each tube to reduce distortion of the amplified signal to a low level. The amount of quiescent current allowed to flow is obviously very important to maintain optimum operating conditions. Too much current will cause harmonic distortion, increase heat dissipation in the anode, reduce audio output power and reduce the operational life of the tube. Too little current will cause severe harmonic distortion and, under extreme conditions, could cause a condition known as ‘cathode poisoning’, which also reduces the effective operation life of the tube.
Conventional Methods of Biasing Fall Mainly into Two Categories
- (1) Cathode (or ‘automatic’) Bias in an attempt to automatically control the quiescent current flow in the tube, a high power resistor is connected in the cathode circuit to signal ground. The grid is connected via a high tube of resistance (usually 500k Ohms or greater) to earth or signal ground. The principle of operation is that when current starts to flow through the tube (and the cathode resistor), a volt drop, proportional to the current will occur across the resistor.This will effectively make the cathode more positive than the signal ground (control grid potential). In effect, this is exactly the same as making the control grid more negative than the cathode. The grid will therefore begin to restrict the flow of current until equilibrium is obtained. The value of this current will be proportional to the value of cathode resistance. Any tendency for the current to increase or decrease due to aging or external conditions will be countered by the resultant change in control grid voltage.
Disadvantages of Cathode Biasing
Cathode biasing has been used in audio amplifier circuits for many years, and although fairly effective, it also has several serious drawbacks notably:
- In high power amplifiers, the value of the cathode resistor must be made relatively large, which results in them needing to dissipate several Watts of heat (adding to the general heat dissipation within the amplifier chassis).
- Reduced operating efficiency.
- A significant amount of audio signal power is ‘lost’ in the resistor, thereby reducing the audio output power for any given input power.
- It is necessary to bypass the cathode resistor with a capacitor in order to preserve the audio amplifying properties of the tube. This capacitor is in the audio signal path and therefore has a restricting effect on certain frequencies.
(2) Fixed Bias
Higher power amplifiers tend to use fixed bias due to the disadvantages of cathode biasing already mentioned. It has a tendency to be used in guitar amplifiers where distortion due to maladjustment comes second to higher output power. Fixed bias incorporates an external negative supply to the grid, which is controlled by a manually adjusted trimmer resistor. It is common practice to supply more than one of the tubes from the same adjustment.
Disadvantages of Fixed Biasing
Setting the bias conditions requires technical knowledge and skill; therefore this operation should be performed by an audio technician.
- Once set any tendency for the current to increase or decrease due to tube aging or external conditions cannot be catered for, therefore conditions can only be optimum immediately after adjustment.
- Frequent adjustments are necessary throughout the life of the valves to preserve optimum performance.
- If a single bias control is used, at best one, or possibly none of the output valves will operate under optimum conditions.
- Maladjustment of the bias control or slight leakage in grid coupling capacitors could result in severe (very costly) damage to the power supply, power output tubes or output transformer.
Heating Effect
It is a misnomer to assume that because tubes rely on thermionic emission for their normal operation, that they cannot overheat. Power output tubes are especially vulnerable to the effects of overheating due to their high anode dissipation under normal driven conditions. Any extra heat generated due to the effects of faulty coupling components, incorrect biasing or poor design can seriously shorten their operational life. Common conditions which cause overheating are:
- Faulty (high leakage) grid coupling capacitors – if cathode bias is used this can be offset to some degree by the automatic compensating effect of the cathode resistor, although overheating and destruction of the resistors may occur before audio distortion is noticed by the listener. In a fixed bias system, it is almost certain that severe damage will result.
- Failure of the cathode resistor by-pass capacitor in cathode bias systems.
- Bias voltage maladjustment in ‘fixed bias’ systems.
- Gas in the valve envelope (‘soft valve’)
Thermionic Tubes & Tube Amplifier Operation
Friday, June 12th, 2009
Thermionic tubes
Thermionic tubes are very tolerant if their specified operating conditions are temporarily exceeded; unlike most semiconductors which tend to suffer from ‘catastrophic’ failure (defined as a failure mode which causes instantaneous and irreversible damage).Unlike semiconductors, tubes do however, ‘wear out’, which means that they have a finite operating life.
Operating them under unfavourable conditions or beyond the manufacturers working specification, usually results in a reduced lifespan, although this may not be audibly apparent in their operation. In audio applications, the majority of seriously damaging problems lie in the power output stages.
Introduction to Tube Amplifier Operation
The majority of amplifiers use at least two output tubes in a circuit configuration known as ‘Push-Pull’. Basically, this means that the audio signal is split into two halves by preceding circuits. Each of the two ‘Split’ signals now drives one output tube.
The signals then undergo power amplification by the output tubes and are re-combined by a special audio grade transformer which also drives the loudspeaker. Please note that, although there are many different types of tube for various applications, the vast majority operate on the same basic principles.
Tubes & Their Internal Construction
Thursday, June 11th, 2009
Tube Internal Construction
Tube Construction
Cathode – this is the ‘power house’ of the tube as it is coated with a material which, when heated, releases electrons. The entire operation of the tube relies on controlling their movement (hence the term ‘Thermionic’).
Heater – this is an electrically heated filament which provides thermal stimulus to the cathode.
Grid – depending on the type of tube, their may be one or more grids present, each with a specific function, however all audio tubes contain at least one control grid. The control grid is made negatively charged with respect to the cathode in order to restrict or prevent the flow of electrons in the valve.
Anode – this electrode is made positive with respect to the cathode (usually by a few hundred volts), and serves to collect the electrons which are allowed to pass through the grid.
Vacuum – audio grade tubes will only operate satisfactorily if there are virtually no air or gas molecules in the envelope. To achieve this, the glass envelope is evacuated to a very high degree during manufacture, which is known as a ‘hard’ vacuum. If small amounts of gas are present, their molecules will ‘ionise’ due to the high voltages present and can give rise to electrical noise or overheating in worse cases. Under these conditions, the valve is said to have ‘gone soft’. Each valve contains a small amount of a reactive metal such as barium (known as the ‘getter’), which is burnt inside the envelope after evacuation in order to ‘mop-up’ the last remaining gas molecules. If the envelope is cracked or broken, air will enter and cause the heater to oxidise and ‘burn out’; it will also disrupt the electron flow and the tube will cease to conduct current. It will be seen from the description of the control grid that if an alternating potential audio signal is applied to the grid, it will modulate the flow of electrons to the anode. Electrical current is defined as electron flow; therefore the signal applied to the grid will produce a modulated current in the anode circuit. Under ideal conditions, this current will be an exact replica of the input signal, but of much greater amplitude. Power valves are optimised to produce large current swings at the anode for relatively small control grid signals. This large output current is used to drive a loudspeaker via a matching transformer.